Data Communications
LAN Systems
ALOHA
Packet Radio
When station has frame, it sends
Station listens (for max round trip time)plus small
increment
If ACK, fine. If not, retransmit
If no ACK after repeated transmissions, give up
Frame check sequence (as in HDLC)
If frame OK and address matches receiver, send ACK
Frame may be damaged by noise or by another station
transmitting at the same time (collision)
Any overlap of frames causes collision
Max utilization 18%
Slotted ALOHA
Time in uniform slots equal to frame transmission
time
Need central clock (or other sync mechanism)
Transmission begins at slot boundary
Frames either miss or overlap totally
Max utilization 37%
CSMA
Propagation time is much less than transmission time
All stations know that a transmission has started almost
immediately
First listen for clear medium (carrier sense)
If medium idle, transmit
If two stations start at the same instant, collision
Wait reasonable time (round trip plus ACK contention)
No ACK then retransmit
Max utilization depends on propagation time (medium
length) and frame length
Longer frame and shorter propagation gives better utilization
If Busy?
If medium is idle, transmit
If busy, listen for idle then transmit immediately
If two stations are waiting, collision (1-persistent
algorithm)
With non-persistent algorithm, if medium is busy,
go away and come back later
P-persistent algorithm – if medium busy, wait;
when idle, transmit with probability p (backoff
with probability 1-p); p = 0.1 or 0.01
CSMA/CD
With CSMA, collision occupies medium for duration
of transmission
Stations listen while transmitting
If medium idle, transmit
If busy, listen for idle, then transmit
If collision detected, jam then cease transmission
After jam, wait random time then start again
Binary exponential back off
Collision Detection
On baseband bus, collision produces much higher
signal voltage than signal
Collision detected if cable signal greater than
single station signal
Signal attenuated over distance
For twisted pair (star-wired bus) activity on more
than one port is collision
IEEE 802.3 Frame Format
Ethernet (CSMA/CD)
Carrier Sense Multiple Access with Collision
Detection
Xerox created Ethernet back in the 70s
IEEE 802.3 became the standard
802.3 and Ethernet are not exactly the same
10Mbps Specification
(Ethernet)
<data rate><Signaling method><Max segment length>
Medium:
Signaling:
Topology:
Nodes:
10Base5
10Base2
10Base-T
10Base-FP
Coaxial
Baseband
Manchester
Bus
100
Coaxial
Baseband
Manchester
Bus
30
UTP
Baseband
Manchester
Star-wired bus
<100
850nm fiber
Manchester
On/Off
Star-wrd bus
33
100Mbps (Fast Ethernet)
100Base-TX
2 pair, Cat 5 UTP
4B5B, MLT-3
100Base-FX
100Base-T4
2 optical fiber
4B5B, NRZI
4 pair, cat 3,4,5
8B6T, NRZ
Also known as IEEE 802.3u.
100Base-TX most popular, uses same RJ-45
connectors and pin configuration as 10BaseT.
100Mbps (Fast Ethernet)
EIA/TIA recommends 90m between end-points (to
allow for 10m in jumper cables) with 100BaseTX.
100Base-FX can have 412m segments using
62.5/125 micron fiber.
1000 Mbps (Gigabit) Ethernet
IEEE 802.3z standard for fiber, 802.3ab for twisted
pair
Data transfers over cables at 1 billion bps
In 1998, prices per port for 10Mbps - $212;
100Mbps - $432, 1000Mbps - $2,200.
In 2003, prices per port for 10Mbps - $12;
100Mbps - $35; 1000Mbps - $100.
Gigabit Ethernet - Potential
Gbps throughput across network backbone
between switches
Gbps throughput between backbone switches and
file servers
Improved workstation and client/server application
performance
Gbps throughput across the client end of a small
business network
Gigabit Ethernet –
Potential Drawbacks
Different patch cable formats may cause some
interconnection difficulties
Network devices that process TCP/IP packets in
software as opposed to in chips may create
bottleneck situations
Gigabit Ethernet not supported everywhere yet
Need Cat 5e or better
NICs need to be PCI-X or 64-bit bus architecture
Gigabit Ethernet Configuration
Gigabit Ethernet - Physical
1000Base-SX
Short wavelength, multimode fiber
1000Base-LX
Long wavelength, Multi or single mode fiber
1000Base-CX
Copper jumpers <25m, shielded twisted pair
1000Base-T
4 pairs, cat 5/5e UTP
Signaling - 8B/10B
Gigabit Ethernet – Other Terms
Backpressure – a flow-control technique that
avoids frame loss by impeding external traffic
from sending frames to congested interfaces
GBIC (gigabit interface converter) – type of hotswappable, standards-based transceiver used
with switches. It converts electrical signals to
optical signals and vice versa, and is usually
used for connections on or to the backbone.
VLAN – Virtual LAN – logical grouping of nodes
regardless of their physical location
Gigabit Ethernet – Other Terms
GVRP (Generic VLAN Registration Protocol) – A
specification defining parameters that switches
use to exchange information for registering
VLANs on a Spanning-Tree network and to
facilitate direct communication
With GVRP, the switch can exchange VLAN
configuration information with other GVRP
switches, prune unnecessary broadcast and
unknown unicast traffic, and dynamically create
and manage VLANs on switches connected
through 802.1Q trunk ports.
Gigabit Ethernet – Other Terms
IEEE 802.1p – A standard that provides quality of
service in Ethernet networks. It relies on packet
tags and allows switches to transmit packets in
order of priority. These tags are in the 802.1q
header.
Possible priorities are 7 – network control; 6 –
voice; 5 – video; 0 – best effort
Gigabit Ethernet – Other Terms
IEEE 802.1q – A standard that defines Ethernet
frame tags that carry VLAN identifiers
The tag is inserted into the CSMA/CD frame
between the SA and Ethertype/Length fields
Tag is 4 bytes – first 2 bytes is Tag Protocol
Identifier. Next 3 bits is Priority field (802.1p).
Next bit is Canonical Format Indicator (used in
Token Ring and FDDI). Final 12 bits are VLAN
ID.
Gigabit Ethernet – Other Terms
IEEE 802.3x – A standard that defines Ethernet
frame start and stop requests and timers on fullduplex links; flow control dialog
Port mirroring – A transmission method in which
frames transmitted and received on one port
can be duplicated on another port for diagnostic
purposes
Port trunking – A link aggregation technique that
creates a single, high-speed logical link from
several lower-speed physical links
Token Ring (802.5)
MAC protocol
Small frame (token) circulates when idle
Station waits for token
Changes one bit in token to make it SOF for data frame
Append rest of data frame
Frame makes round trip and is absorbed by
transmitting station
Station then inserts new token when transmission has
finished and leading edge of returning frame arrives
Under light loads, some inefficiency
Under heavy loads, round robin
Token Ring
Operation
Token Ring MAC Frame
Priority
Scheme
802.5 Physical Layer
Data Rate
Medium
Signaling
Max Frame
Access Control
4
16
100
UTP,STP,Fiber
Differential Manchester
4550
18200
18200
TP or DTR TP or DTR DTR
Note: 1Gbit in development (yeah, right)
FDDI
100Mbps
LAN and MAN applications
Token Ring on steroids
FDDI MAC Frame Format
FDDI MAC Protocol
Same as 802.5 except:
Station seizes token by aborting token transmission
Once token captured, one or more data frames
transmitted
New token released as soon as transmission finished
(early token release in 802.5)
FDDI
Operation
FDDI Physical Layer
Medium
Data rate
Signaling
Max repeaters
Between repeaters
Optical Fiber
100
4B/5B/NRZI
100
2km
Twisted Pair
100
MLT-3
100
100m
Fiber Channel - Background
I/O channel
Direct point to point or multipoint comms link
Hardware based
High Speed
Very short distance
User data moved from source buffer to destination
buffer
Network connection
Interconnected access points
Software based protocol
Flow control, error detection &recovery
End systems connections
Fiber Channel
Best of both technologies
Channel oriented
Data type qualifiers for routing frame payload
Link level constructs associated with I/O ops
Protocol interface specifications to support existing I/O
architectures
e.g. SCSI
Network oriented
Full multiplexing between multiple destinations
Peer to peer connectivity
Internetworking to other connection technologies
Fiber Channel Elements
End systems - Nodes
Switched elements - the network or fabric
Communication across point to point links
Fiber Channel Network
Fiber Channel Protocol
Architecture (1)
FC-0 Physical Media
Optical fiber for long distance
coaxial cable for high speed short distance
STP for lower speed short distance
FC-1 Transmission Protocol
8B/10B signal encoding
FC-2 Framing Protocol
Topologies
Framing formats
Flow and error control
Sequences and exchanges (logical grouping of frames)
Fiber Channel Protocol
Architecture (2)
FC-3 Common Services
Including multicasting
FC-4 Mapping
Mapping of channel and network services onto fiber
channel
e.g. IEEE 802, ATM, IP, SCSI
Review Questions
If 100 workstations are connected to a CSMA/CD
LAN with a 0.01 persistence algorithm, how
many stations might try to transmit at the
same time?
In what type of scenario would we use CSMA over
CSMA/CD?
What is the Pad field (in CSMA/CD) used for?
Why does a token ring station take its own data
off the ring?
Why is the ED field in token ring after the CRC?